Acetylene can decompose explosively (i.e., decompose into carbon and hydrogen) under conditions of high pressure and temperature, even in the absence of air or oxygen. Therefore, acetylene cylinders are constructed uniquely. Each cylinder contains porous filler with solvent such as acetone, dimethylformamide (DMF) or N-methylpyrrolidone (NMP) distributed throughout the filler. The filler is a porous mass generally having a porosity of around 90% by volume. The function of the porous filler is to separate acetylene into small units in the pores that help to inhibit the decomposition of acetylene. The function of the solvent is to absorb large amounts of acetylene at relatively low pressures to enable high cylinder loading in low pressure cylinders.
Due to their unique construction, the preparation of acetylene cylinders for high purity acetylene is a challenge. For example, with regard to the porous filler material, impurity gases such as nitrogen, oxygen and carbon dioxide can be trapped inside the pores during the formation of the porous mass and during subsequent cylinder leak testing with air or nitrogen. Typically, acetylene cylinders will be vacuumed down to around 24 inch Hg to remove the majority of air impurities prior to solvent charging. In conventional techniques, solvent is then charged directly into the cylinder without any prior degassing step. Dissolved gases, such as nitrogen, oxygen and the like, are at a level of several hundred parts per million (ppm) in the solvent at room temperature conditions which could accumulate in the cylinder headspace after solvent and/or acetylene charge. The cylinders that are treated in this way may easily contain air impurities up to a few percent after the initial acetylene charge. This is not an issue for industrial grade acetylene.
Recently, acetylene has been used increasingly as a source material for depositing carbon and carbon-containing films in the electronic industry. Applications include the deposition of amorphous carbon hard mask films. High levels of atmospheric impurities or any other impurities in acetylene may reduce the film deposition rate and affect process uniformity and consistency. For such applications, less than about 100 parts per million (ppm), oftentimes less than about 10 ppm, of atmospheric impurities in acetylene may be desirable.
Therefore, a need exists for preparing acetylene cylinders for high purity acetylene storage, transport and/or delivery. Particularly, a need exists for preparing acetylene cylinders for use with high purity acetylene that can be used in the semiconductor industry. It would be desirable in the art to develop improved methods for preparing acetylene cylinders for high purity acetylene storage.